It depends on how many SG characters are canonically fathered by WIAB William. At this point, Ayua is the only strong candidate. Everyone else is speculation with no real evidence.I’ve read what you all said, but isn’t WIAB a game with multiple love interests? So you choose who William ends up with, which means nothing is really set in stone when it comes to who’s the parent of who in Summer’s Gone. Unless there’s one canon ending in WIAB and the rest are just alternate universes where you pick the LI you want… of course, a harem ending would fix all of this.
Then, it depends on how WIAB plays out. Just because William potentially ends up with a particular LI at the end of the game doesn't preclude him manwhoring progeny into this world with multiple women along the way. The downside is that would involve forced pregnancy to kept both AVNs aligned. At this point, the only likely forced pregnancy would be with Miru because of Ayua, and WIAB has already established that Miru wants to have William's babies, hell or high water.
A real killer plot twist would be if William turns out to be impotent.